Good idea, bad implementation Rated 3 out of 5 stars
This is the correct way to handle groups, unlike Mozilla's version which hides inactive groups in such an odd UI. (For them, just using separate Windows makes more sense.)
But, unfortunately, it does not play well with tabs on top. It needs to take the top spot, and have tabs right below it, and then the address bar. Having the address bar in the title bar just looks awful and subverts the rationale of having tabs on top.
It also would be a good idea to have a mode where the tabs stay on top, and the groups go just below. That way you still get the other advantage of tabs on top: faster navigation, as you can move the mouse up to the top really quickly and you'll always be on the tab bar.